Increase in CO2 emissions, such as the combustion of fossil fuels, has contributed to climate change. As a result, measures to reduce CO2 emissions are needed. Carbon capture and storage (CCS) has been proposed as a major climate change mitigation technology that may capture up to 90% of the CO2 emissions from fossil fuel-fired facilities.
CCS processes consist of three stages: CO2 capture, transportation, and storage. The first stage is the most challenging due to the high cost of currently available technolgies.
Oxy-fuel combustion is a technology used for capturing CO2 from large-scale fossil fuel-fired facilities with the potential to reduce CO2 emissions and meet CCS requirements. In oxy-fuel combustion, fuel is combusted in O2/CO2 atmosphere producing a CO2-rich flue gas with some impurities such as SOx, NOx, Hg and H2O. When the combustion proceeds with a fluidized bed system, such as oxy-FBC (fluidized bed combustion), there are inherent advantages, e.g., fuel flexibility, moderate combustion temperature, and low impurity generation. Further, fluidized bed configuration enables in-situ SO2 capture via sulphur removing sorbents, such as limestone or dolomite, resulting in reduced corrosion risk to system components caused by acid attack after the formation of SO3 at high partial pressures of SO2 and steam.
Oxy-fuel fluidized bed combustion has been shown to be a clean energy technology that can utilize a variety of fuels for producing steam and electrical power and is now demonstrated at the 30 MWth scale and is available at the 330 MWe scale for demonstration. The pure stream of carbon dioxide that can be geologically sequestered; thereby, eliminating the emission of greenhouse gases resulting from combustion of fuels.
There still remain certain drawbacks with the current oxy-fuel fluidized bed combustion technology.
Typically, oxy-fuel fuel combustors use recycled flue gas to provide fluidizing gas and to provide a temperature moderator within the combustor. This flue gas stream requires substantial capital investment and imposes a significant parasitic power loss on the combustor facility.
A major portion of the heat released during combustion is extracted using an in-bed heat exchanger (located within the fluidized bed). Conventional fluidized bed combustion technologies use inert solid particulate substances as the bed material or calcium bearing sulphur capture sorbents. Over time the bed material attrites and forms finer material that is blown out of the fluidized bed, and at which time a bed material replenishment system is required.
While fluidized bed combustors are considered to exhibit good mixing characteristics, they do not mix gases radially to the extent as desired, resulting in regions in the fluidized bed where reducing conditions prevail. The reducing zones result in increased emissions of carbon monoxide and other deleterious impurities and cause corrosion to the boiler components such as boiler tubes, tube supports, and injection ports. For example, there can be localized regions where there is insufficient oxygen to fully combust the fuel. The products of incomplete combustion include the products of complete combustion, as well as a variety of reduced species including hydrogen (H2), carbon monoxide (CO), hydrogen sulphide (H2S), methane (CH4), higher hydrocarbons, and ammonia (NH3). These species are not desirable in the flue gas of a combustor. It is possible for these species, if in sufficient quantity, to later mix with oxygen resulting in explosions within the downstream equipment. The metal alloys used in components in the combustor typically have a protective oxide layer that prevents corrosion (stainless steel is ‘stainless’ due to this oxide layer); however, these species are able to reduce the metal oxide layer thereby eliminating the protective layer resulting in corrosion of the components. Corrosion of these components can result in increased erosion rates and component failure. Incomplete combustion reduces the amount of heat that can be recovered from the combustor, since a portion of the fuel has not been burnt and therefore the overall efficiency of the system is reduced. Sulphur removal sorbent performs best when sulphur oxides and oxygen are present. A number of the reduced species can reduce the effectiveness of the CO2 processing unit since the removal of these impurities is required to meet pipeline specifications.
To limit or avoid localized regions with insufficient oxygen, large scale combustors incorporate a multitude of fuel and oxidant injection points. As the number of injection points increases, the cost and complexity of the combustion system also increase.
Several studies have focused on improving fluidized bed combustion processes using reactive bed materials. These improvements include reduced emissions of unburned hydrocarbons, enhanced sulphur capture, improved NOx reduction, increased agglomeration resistance, and reduced corrosion issues.
For example, U.S. Pat. No. 4,084,545 (Nack et al.) describes a method of operating a fluidized bed combustion system comprising an entrained fluidized bed of fine particles part of which also contains a non-entrained fluidized bed portion of coarse particles. The fine particles pass out of the bed with the fluidizing gas, pass through a gas-particle separation device and are then re-entrained into the lower portion of the entrained bed. The coarse particles are retained in the fluidized bed. The fine particles are suggested to be hematite ore, limestone, aluminum oxide, nickel or nickel oxide with their primary purpose being rapid heat transfer to boiler components. It is recognized that redox reactions may occur if the bed material contains iron or nickel though the material has not been selected to enhance this effect. It is also recognized that calcium containing compounds can be used to capture sulfur species. This patent discusses an air-fired combustion system operating at approximately atmospheric pressure. The fluidizing gas is indicated to be in the range 6-12 meters per second which exceeds reasonable limits required to avoid boiler tube erosion in a fluidized bed combustion system with velocities less than about 1.2 meters per second being most appropriate. The high fluidizing velocity has in part been specified due to the fairly large size of the fine particles (420 to 841 micron). It is assumed that the fine particulates will be recycled into the entrained bed, but this patent provides no method of separating fuel ash components from the intended fine particles used for heat transfer. The purpose of the coarse bed particles is to restrict the movement of the coal particles in the principal direction of air flow as to increase the residence time of the coal particles. The coarse particles are specified to be chemically stable (i.e. inert).
U.S. Pat. No. 4,154,581 (Nack et al.) provides an extension to U.S. Pat. No. 4,084,545 through the inclusion of a baffle in the dense bed region to separate the bed into two separate regions operating at differing temperatures to promote the adsorption of sulphur by a calcium bearing sorbent at one temperature while promoting the rate of combustion at the second temperature, although not addressing the other drawbacks of the '545 patent as noted above.
I Adanez-Rubio et al. (Fuel Processing Technology, 2014, 124, 104-114) described a system for oxidizing biomass through the use of chemical looping with dual fluidized beds in which one bed acts as an oxygen carrier oxidizer (the air reactor) and one bed gasifies the biomass (the fuel reactor).
Thunman et al. (Fuel, 2013, 113, 300-309) found that by introducing ilmenite (Fe—Ti based oxygen carrier) to a 12 MWth circulating fluidized bed (CFB) boiler for biomass combustion, the concentrations of CO, NO, and hydrocarbon were reduced significantly. This was attributed to enhanced oxygen distribution throughout the bed via intermittent reduction and oxidation of ilmenite causing variations in oxygen partial pressures in different regions of the combustor.
Corcoran et al. (Energy Fuels, 2014, 28, 7672-7679) noted that the structure of ilmenite particles injected into a CFB boiler for biomass combustion experienced structural and chemical changes due to the diffusion of potassium from ash into the core of the ilmenite particles. This was found to improve the bed material agglomeration resistance and reduce corrosion issues.
D. R. Chadeesingh et al. (Fuel, 2014, 127, 169-177) discussed that the introduction of an iron-based oxygen carrier into a bubbling fluidized bed for CH4 combustion with air has also been shown to accelerate the combustion of CH4, CO and H2.
Still, there remains the need to address the drawback and risks associated with the current state of art as summarized below in Table 1.
TABLE 1Risks of Oxy-Fluidized Bed Combustion Systems and Mitigation MeasuresRisks associated with the Current State ofArt In Oxy-Fluidized Bed CombustionConventional Mitigation MeasuresSulphur capture using calcium basedChange sorbent particle size, change sorbentsorbents is insufficient to meet CO2source, use alternate sulphur capture technologytransportation specificationsIncreased O2 partial pressure will enhancesorbent sulphur captureAgglomeration, deposition and fouling ofChange recycle gas ratio to control temperatureash and sorbent on boiler tubes and processof the combustor, change oxygen staging toequipmentavoid reducing conditions in regions that areaffectedMore uniform combustion would minimizelocal “reducing environment” and “hotspots,”reducing the risk of agglomeration, depositionand foulingCorrosion and erosion of fuel injectionDependent on local O2 partial pressure -components and boiler tubestherefore fuel injection and dispersion test workto ensure O2 partial pressure controlledMaterials of constructionModify geometry to reduce particle velocities inaffected areas
As can be readily appreciated by a person skilled in the art, there is a trade-off in technical and economic performance of the system based on the ability of the system to transfer sufficient oxygen to complete combustion throughout the combustion region.
It is desirable then to develop a technology which meets this objective without substantially affecting cost or reliability of the system.
While oxy-FBC technology operating at atmospheric pressure has been a technical success, the cost of power is still seen as a barrier to deployment of the technology.
To reduce cost and increase efficiency of oxy-fuel systems, pressurized oxy-combustion technologies are being developed that will have efficiencies 15 to 25% higher than their atmospheric pressure equivalents, reducing the cost of power of at least 20% for power generation with CCS.
As a result, there remains the need to reduce the size of the recycle stream which will improve the economic outlook for deploying oxy-solid fuel combustors technology.